This invention relates to a drive control method for a stepping motor used for driving a printing head, and more particularly a drive control method for a stepping motor driver of the slow-decay type.
There is proposed a method for driving and controlling a printing head of a printer by use of a slow-decay type of a stepping motor.
A drive circuit for the stepping motor is of the bipolar type in which to efficiently use the coils, each motor phase is operated by use of a bridge circuit and the polarity of the motor phase is alternately inverted by flowing bipolar current thereto.
FIGS. 8 and 9 show a drive circuit for driving a slow-decay type of a stepping motor. As shown, a bridge circuit is formed by use of four diodes D1 to D4. Switching transistors Q1 to Q4 are coupled across those diodes, respectively. To set up a chopping mode in the bridge circuit, switching transistors Q1 and Q4 are turned on, and current flows through a current path including a coil L (FIG. 8). To remove the chopping mode, the switching transistor Q4 remains on, and the switching transistor Q1 is turned off. As a result, current supply to the coil L is stopped. At this time, the coil L generates an electromotive force because of electromagnetic nature thereof, so that current flows through the coil L, the switching transistor Q4, and the diode D2 in this order (FIG. 9). As a result, the current flowing through the coil L is gradually attenuated. The same thing is true for a case where the coil L is energized by the transistors Q2 and Q3.
The conventional drive circuit for the stepping motor of the slow-decay type uses a W1-2 phase driving method (as shown in FIG. 7) for the motor speed control in all acceleration, constant-speed and deceleration states or conditions.
In FIG. 7, a signal PH1 is a signal indicative of a polarity of current flowing through a first phase coil; signals I01 and I11 are step pulses for controlling the current flowing through the first phase coil; a signal OUT1 is a current output to the first phase coil; a signal PH2 is a signal indicative of a polarity of current flowing through the second phase coil; signals I01 and I12 are step pulses for controlling the current flowing through the second phase coil; and a signal OUT2 is a current output to the second phase coil. The current waveforms indicated by solid lines shown in OUT1 and OUT2 in FIG. 7 are logic waveforms, and their actual waveforms are as indicated by dashed lines.
The waveform of the current signal OUT1 is configured in four steps by combinations of signal states "H" and "L" of two step pulse signals I01 and I11. Namely, when both of I01 and I11 are "H", OUT1 becomes 0%. When I01 is "L" and I11 is "H", it becomes 33% (or -33%). When I01 is "H" and I11 is "L", it becomes 66% (or -66%). When both of I01 and I11 becomes "L", it becomes 100%. The above combination relationships are correspondingly applied to the combination relationships between the current signal OUT2 flowing into the second phase and the two pulse signals I01 and I02 for controlling it.
As seen from the above description, in the W1-2 phase driving method, when current is supplied to the first phase coil, the output current signal OUT1 is gradually increased (in four steps) up to 100% (or -100%). When current supply to the first phase coil is stopped, the output current signal OUT1 is gradually decreased (in four steps) up to 0%.
More specifically, during a time period from a time point t1 (at which current supply to the first phase coil starts) to a time point t2, the pulse signal I01 is set at "H" and the pulse signal I11 is set at "H", and the output current signal OUT1 is 0%. During the succeeding time period from t2 to t3, the pulse signal I01 is "L" and the pulse signal I11 is "H", and the output current signal OUT1 is 33%. During the succeeding time period from t3 to t4, the pulse signal I01 is "H" and the pulse signal Ill is "L", and the output current signal OUT1 is 66%. During the succeeding time period from t5 to t6, the pulse signal I01 is "L" and the pulse signal I11 is "L", and hence the output current signal OUT1 is 100%. Thus, the output current signal OUT1 is gradually increased from 0% to 100% during a time period from t1 to t4.
During a time period from time points t4 to t7, the output current signal OUT1 is kept at 100%. During a time period from t7 to t8, the pulse signal I01 is "H" and the pulse signal Ill is "L", and then the output current signal OUT1 is 66%. During a time period from t8 to t9, the pulse signal I01 is "L" and the pulse signal I11 is "H", and hence the output current signal OUT1 is 33%. During a time period from t9 to t10, the pulse signal I01 is "L" and the pulse signal I11 is "L", so that the output current signal OUT1 is 0%. Thus, during a time period from t7 to t10, the output current signal OUT1 is gradually decreased from 100% to 0%.
Similarly, when current is also fed to the second phase coil, the output current signal OUT2 is gradually increased (in four steps) to 100% (or -100%). Also when current feeding to the second phase coil is stopped, the output current signal OUT2 is gradually decreased (in four steps) to 0%.
In case where the W1-2 phase driving method is used for the speed control of the printing head, drive sound and vibrations are diminished in particular when the printing head is accelerated (driven to start its movement) and decelerated (stopped).
In case where the W1-2 phase driving mode is performed in the constant-speed state where the printing is carried out by the printing head, the current control fails when the drive circuit for the slow-decay type stepping motor is operated in a microstep driving mode, in particular when the current is attenuated. Comparing an actual waveform (depicted by dashed lines) of the output current signal OUT1 (OUT2) with a logic waveform (depicted by solid lines) of the same in their amplitude decreasing portions. As seen from the comparison, the actual waveform more gently decreases its amplitude than the logic waveform, and after a short time, its amplitude abruptly decreases to 0% while the logic waveform stepwise decreases its amplitude. This output current variation will causes printing head vibrations and vertical stripes which appear in the printed picture or print while being spaced at fixed intervals. Those vertical stripes are a little distinguished in normal print; however, those are likely to occur and becomes problematic in high definition print.
To solve the stripe problem, there is a proposal of a drive control method for a stepping motor (referred frequently to as a motor drive control method), disclosed in Unexamined Japanese Patent Publication No. 5-278293. In the motor drive control method, in acceleration and deceleration states or conditions, the motor speed control progresses selectively using the 1-2 phase driving mode and the 2-2 phase driving mode. More precisely, in low speed states, the 1-2 phase driving mode is used which less produces vibrations and noisy sounds (drive sound). In high speed states, the 2-2 phase driving mode is used which requires a small number of motor-drive switching.
The conventional motor drive control method succeeds in suppressing generation of vertical stripes in a satisfactory level in normal print. However, it has still such a problem that a definition of the print is unsatisfactory in high definition print.